Here you can see the differences between the handguard retaining screw lugs on all three 10.5" SMRs.
Left to Right:
1. 05-190, REV. A
2. 05-173, REV. B
3. 05-190, REV. B
You can see that the lugs are quite similar between the Prototype (05-172), and first production rail (05-190, REV. A) in general shape, but the production rail is slightly larger and contoured slightly differently.
Comparing between the 05-190, REV. A, and REV. B, however, you can see that the lug was drastically enlarged between the REV. A and B, and much more rectangular in shape:
Here you can see the lugs on all four rails side by side:
In addition to the minor changes to the lugs themselves, the handguard screw retainer which keeps the screws semi-captive to the rail have slightly changed between variants as well:
05-172, REV. B:
Interestingly, the Prototype rail appears to have a marking under the lug (which I unfortunately neglected to photograph) where someone has written "roller" by hand in marker, and on my example, which was likely a test unit, it actually moves in the hole when pressed (it does not appear to be intended to move). Most notably, however, this retaining pin appears to be located in the bottom corner of the lug.
Starting with the 05-190, REV. A, this pin was moved to the center of the lug:
This retaining pin appears to have stayed in this position on all subsequent 416 SMRs:
05-190, REV. B:
05-191, REV. B:
Note that on the North Wales produced 05-191, REV. B, a scallop appears to have been added, presumably to ease installation, though that is merely speculation.
Moving on, the Prototype rail also seems quite distinctive in lacking the corner cuts and top cut on the rail's interior:
One fact that I found extremely unexpected and unusual was that while the Prototype rail and forst production rail appear to be identical in length--the 05-190, REV. B was noticeably shorter than the Prototype and REV: A, here they are lined up end to end on the receiver side (you can also compare the handguard screw lugs):
(T to B both photos: 05-190, REV. A; 05-172, REV. B; 05-190, REV. B)
In some ways, the 05-190, REV. A appears to be closer to the 05-172, REV. B than the 05-190, REV. B.
While the 05-190, REV. A production rail introduced the 45 degree QD sling swivel, it is of a different design than the one on the REV. B that appears to be used on all subsequent variants, being thinner, and rotating 360 degrees around, unlike the rotation limited QD on the REV. B (the same swivels will fit in the sockets):
Here you can see the QDs on both the 05-190 and 05-191, REV. Bs compared with the 05-190, REV. A and 05-172, REV. B (no QDs):
You can also see that the numbering for the top rail has remained consistent since the prototype, and you can also see where the front rail profile was shortened and altered between the 05-190, REV. A and REV. B. The front rail profile of the 05-191, REV. B is consistent, albeit longer, with the 05-190, REV. B.
All the Norristown markings together:
My lonely North Wales marked rail:
The 05-190, REV. A was also signed by none other than Bill Geissele, thanks to Charlie, at this year's (2017) SHOT Show:
Hope y'all found this interesting! I will probably add some photographs of these rails mounted on an upper (or at least the 10.5"s, since my other MR556 upper is still out being cut to 14.5" with a permanently pinned Surefire FH556-212A) at a later date, included the 05-190, REV. B "kitted out," (since my REV. A needs to be preserved, I think I'm already beginning to lose the top of the "7"